Field of the Invention
The present invention generally relates to a gas supply system for a reaction chamber, particularly to a method for stabilizing reaction chamber pressure when supplying a gas to the reaction chamber.
Description of the Related Art
In an atomic layer deposition (ALD) process, controlling a flow rate and supply time of material constituting a source gas for forming a film is very important to achieve a good process result and good device stability. When a liquid material or a solid material having a relatively low vapor pressure at ambient temperature is used as source material, a desired vapor pressure can easily be obtained by heating a reservoir (or bottle) containing the material, and pressure fluctuation when supplying the material to a reaction chamber can be suppressed to a insignificant level. FIG. 1 illustrates schematic views of a gas supply system for supplying a source gas having a low vapor pressure to a reaction chamber (RC) 1 according to background art, wherein a gas which does not include a vaporized source gas is supplied to the reaction chamber 1 through a mass flow controller (MFC) 3 and valve v1 provided in a gas line 5 in (a), and a gas which includes a source gas vaporized in a bottle (BTL) 2 is supplied to the reaction chamber through the mass flow controller 3, valve v2 provided in an inlet line 6, the bottle 2, and valve v3 provided in an outlet line 7 in (b). In (a), valve v1 is open, and valve v2 and valve v3 are closed, whereas in (b), valve v1 is closed, whereas valve v2 and valve v3 are open. The pressure in the reaction chamber 1 is controlled during the process by a pressure control valve 4 provided in an exhaust line 9.
However, when a gas material or a liquid material having a high vapor pressure at ambient temperature is used, the material is oversupplied to a reaction chamber because the volume of the material is large. The oversupply of the material brings about adverse effects, e.g., lowered in-plane uniformity of material adsorption on a substrate surface, prolonged purge time to remove the material from the reaction chamber, pressure fluctuation in the reaction chamber, and increased amount of the material used. Therefore, in order to control the amount of supplied material, vapor pressure can be lowered by cooling the material reservoir. However, for that purpose, cooling equipment is installed, and complex hardware and control are required. FIG. 2 illustrates a schematic view of a gas supply system for supplying a source gas having a high vapor pressure to a reaction chamber 1 according to background art. This gas supply system in FIG. 2 is substantially similar to that illustrated in FIG. 1 wherein the same reference numbers indicate the same parts as in FIG. 1, except that a cooling jacket 8 is installed around the bottle 2 so that the material having a high vapor pressure does not generate excessive vapor.
There are other approaches to handle a material having a high vapor pressure. FIG. 3 illustrates schematic views of a gas supply system for supplying a source gas having a high vapor pressure to a reaction chamber according to modified background art, wherein a source gas is supplied to a reaction chamber 1 via a mass flow controller (MFC) 11 through valve v4 provided in a gas line 5 in (a), and a source gas is supplied to a reaction chamber 1 via a MFC 11 and an auto-pressure regulator (APR) 12 through valve v4 and a throttle 13 in (b). In the above, the MFC 11 is installed downstream of the bottle 2 or is provided in a source gas supply line without a bottle where the material is gaseous at room temperature. Since the response speed of the MFC 11 is low, the configuration using the MFC 11 in (a) is insufficient for controlling an advanced ALD cycle, i.e., switching gases in a very short time period (e.g., 0.1 to 1.0 second). The configuration using the MFC 11 and the APR 12 is of a valve-switch-pulsing type wherein flow is controlled by opening and closing the valve in combination with the APR. Since the APR has higher response speed than the MFC, this configuration can be used for an ALD cycle. However, since the pressure in the reaction chamber is increased when the source gas is supplied thereto by operation of the valve and the APR, a large pressure fluctuation occurs as the gas supply starts and ends. The pressure control valve 4 then compensates for the increase of pressure in the reaction chamber by opening the pressure control valve 4. Since the operation of the pressure control valve 4 is not highly responsive and has a delay, the pressure in the reaction chamber is overcompensated. As a result, the risk of particle generation becomes high due to reverse blow from the exhaust line 9 or the like.
FIG. 4 illustrates charts showing changes in source gas flow in the reaction chamber and changes in reaction chamber pressure with time when the gas supply system illustrated in (b) in FIG. 3 is used. The source gas flow is sharply or instantly increased and decreased as shown in FIG. 4. The pressure in the reaction chamber changes as the source gas flow changes as shown in FIG. 4. When the pressure in the reaction chamber increases, the pressure control valve installed downstream of the reaction chamber responds to the pressure increase and starts reducing the pressure by opening the valve so as to maintain the constant pressure in the reaction chamber. However, when the pressure starts lowering due to the operation of the pressure control valve, the source gas flow is stopped, and as a result, the pressure in the reaction chamber is overcompensated and becomes too low due to the response delay. This phenomenon is illustrated in FIG. 4 and marked with an arrow. This type of pressure hunting is not avoidable for systems changing supply gas flow while maintaining the pressure in a reaction chamber.
Any discussion of problems and solutions involved in the related art has been included in this disclosure solely for the purposes of providing a context for the present invention, and should not be taken as an admission that any or all of the discussion were known at the time the invention was made. For example, FIGS. 1-4 (FIGS. 1 and 2 are designated by a legend “Background Art”) illustrate the background of the invention for the present inventors and should not be considered to represent prior art known at the time the invention was made.